1. Field of The Invention
This invention relates generally to aluminum-silicon (Alxe2x80x94Si) alloys. It relates particularly to a high strength Alxe2x80x94Si based alloy suitable for high temperature applications for cast components such as pistons, cylinder heads, cylinder liners, connecting rods, turbo chargers, impellers, actuators, brake calipers and brake rotors.
2. Description of the Related Art
Alxe2x80x94Si alloys are most versatile materials, comprising 85% to 90% of the total aluminum cast parts produced for the automotive industry. Depending on the Si concentration in weight percent (wt. %), the Alxe2x80x94Si alloy systems fall into three major categories: hypoeutectic ( less than 12% Si), eutectic (12-13% Si) and hypereutectic (14-25% Si). However, most prior alloys are not suitable for high temperature applications because their mechanical properties, such as tensile strength and fatigue strength, are not as high as desired in the temperature range of 500xc2x0 F.-700xc2x0 F. To date, many of the Alxe2x80x94Si cast alloys are intended for applications at temperatures of no higher than about 450xc2x0 F. Above this temperature, the major alloy strengthening phases such as the xcex8xe2x80x2 (Al2Cu) and Sxe2x80x2 (Al2CuMg) phase will become unstable, rapidly coarsen and dissolve, resulting in an alloy having an undesirable microstructure for high temperature applications. Such an alloy has little or no practical application at elevated temperatures because, when the xcex8xe2x80x2 and Sxe2x80x2 become unstable, the alloy lacks the lattice coherency between the aluminum solid solution lattice and the strengthening particles lattice parameters. A large mismatch in lattice coherency contributes to an undesirable microstructure that can not maintain excellent mechanical properties at elevated temperatures.
One approach taken by the prior art is to use fiber or particulate reinforcements to increase the strength of Alxe2x80x94Si alloys. This approach is known as the aluminum Metal Matrix Composites (MMC) technology. For example, U.S. Pat. No. 5,620,791 relates to an MMC comprising an Alxe2x80x94Si based alloy with an embedded a ceramic filler material to form a brake rotor for high temperature applications. An attempt to improve the high temperature strengths of Alxe2x80x94Si alloys was also carried out by R. Bowles, who has used ceramic fibers to improve tensile strength of an Alxe2x80x94Si 332.0 alloy, in a paper entitled, xe2x80x9cMetal Matrix Composites Aid Piston Manufacture,xe2x80x9d Manufacturing Engineering, May 1987. Another attempt suggested by A. Shakesheff was to use ceramic particulate for reinforcing Alxe2x80x94Si alloy, as described in xe2x80x9cElevated Temperature Performance of Particulate Reinforced Aluminum Alloys,xe2x80x9d Materials Science Forum, Vol. 217-222, pp. 1133-1138 (1996). Cast aluminum MMC for pistons has been described by P. Rohatgi in a paper entitled, xe2x80x9cCast Aluminum Matrix Composites for Automotive Applications,xe2x80x9d Journal of Metals, April 1991. It is noted that the strength for most particulate reinforced MMC materials, manufactured from an Alxe2x80x94Si alloy, are still inferior for high temperature applications because the major xcex8xe2x80x2 and Sxe2x80x2 strengthening phases are unstable, rapidly coarsen and dissolve at high temperatures.
Another approach taken by the prior art is the use of the Ceramic Matrix Composites (CMC) technology. For example, W. Kowbel has described the use of non-metallic carbon-carbon material for making pistons to operate at high temperatures in a paper titled, xe2x80x9cApplication of Net-Shape Molded Carbon-Carbon Composites in IC engines,xe2x80x9d Journal of Advanced Materials, July 1996. Unfortunately, manufacturing costs employing these MMC and CMC technologies are substantially higher than those using conventional Alxe2x80x94Si casting, which has hampered their ability to be priced competitively with Alxe2x80x94Si alloys in mass production for high temperature internal combustion engine parts and brake applications.
It is accordingly a primary object of the present invention to obviate the disadvantages of the prior art technologies.
According to the present invention, an Alxe2x80x94Si alloy containing dispersion of particles having L12 crystal structure in the aluminum matrix is presented. The alloy is processed using low cost casting techniques such as permanent mold, sand casting or die casting.
The alloy of the present invention maintains a much higher strength at elevated temperatures (500xc2x0 F. and above) than other prior art alloys, due to a unique chemistry and microstructure formulation. The methods for strengthening the alloy in the present invention include: 1) Maximizing the formation of major strengthening xcex8xe2x80x2 and Sxe2x80x2 phase in the alloy, with chemical composition given as Al2Cu, Al2CuMg, respectively. 2) Stabilizing the strengthening phases at elevated temperatures by controlling the Cu/Mg ratio and by the simultaneous addition of Titanium (Ti), Vanadium (V) and Zirconium (Zr) elements. 3) Forming A13X (X=Ti, V, Zr) compounds with L12 crystal structures for additional strengthening mechanisms at elevated temperatures.
In the present invention, key alloying elements of Ti, V and Zr are added to the Alxe2x80x94Si alloy to modify the lattice parameter of the aluminum matrix by forming compounds of the type Al3X having L12 crystal structures (Xxe2x95x90Ti, V and Zr). In order to maintain high degrees of strength at high temperatures, both the aluminum solid solution matrix and the particles of Al3X compounds should have similar face-centered-cubic (FCC) crystal structures, and will be coherent because their respective lattice parameters and dimensions are closely matched. When the condition of substantial coherency for the lattice is obtained, these dispersion particles are highly stable, which results in high mechanical properties for the alloy during long exposures at elevated temperatures.
In addition to the alloy composition and microstructure, a unique heat treatment schedule is provided in order to optimize the performance for the alloy strengthening mechanisms and phases formation within the alloy. The advantages of the present invention will become apparent as the description thereof proceeds.